1. Field of the Invention
The present invention relates to data receivers for receiving high data rate signals from long lengths of cable, and in particular, data receivers for receiving high data rate, baseband, binary or MLT3 encoded data signals from long lengths of cable, while providing adaptive equalization, dynamic data slicing and signal baseline restoration.
2. Description of the Related Art
Recovering data which has been transmitted over a long length of cable at high rates requires that such data be equalized in order to compensate for the loss and phase dispersion of the cable. Further, in those applications where the cable length may vary, such equalization must be based upon a complementary transfer function which is capable of adapting accordingly since the transfer function of the cable varies with the length of the cable. This equalizing is generally done using three functions: a filter function; a dc restoration and slicing function; and an adaptation control, or servo, function.
The filter function is performed using a complementary (with respect to the complex cable loss characteristic) filter which synthesizes the inverse of the transfer function of the cable. Since the bit error rate (BER) is directly related to jitter, an important performance metric for an equalizer is jitter within the output waveform. The extent to which the equalizer is able to match the inverse of the complex cable loss characteristic determines the extent to which inter-symbol interference induced jitter is eliminated.
As for the dc restoration and slicing function, it is well known that ac coupling a digital data stream with variations in pattern density creates baseline wander. If the waveform has finite rise times, then the baseline wander will cause jitter which results in slicing of the data at different amplitude points along the waveform edges, and the finite rise and fall times of such edges translate the amplitude slicing variations to timing variations. One conventional technique for eliminating baseline wander is to use a quantized feedback circuit in which positive feedback around the comparator is used so that very little charging current flows through the input ac coupling capacitor. In other words, the comparator provides its own dc restoration. However, there is a start-up problem associated with the use of quantized feedback. For example, if the comparator starts out in a state opposite that of the input state, then the output may never transition between states at all because the ac coupled input never crosses the comparator threshold. This situation is aggravated by sparse data patterns.
As for the adaptation control, or servo, function, conventional adaptive equalizers use a simple peak detection technique in which a control voltage is produced which is always proportional to the pulse height of the equalized data signal. However, such a peak detection servo is very sensitive to amplitude errors in the incoming data signal.